Ultrathin polyether sulfone films and their preparation

ABSTRACT

Disclosed herein are free-standing, pinhole-free, ultrathin polyether sulfone films having thicknesses of about 400 angstroms or less and a process for preparing them. The films find particular utility in separatory applications.

STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSOREDRESEARCH AND DEVELOPMENT

This invention was made with Government support under Contract NumberDAAK 20-84-C0147 awarded by the Department of Defense (DOD). TheGovernment has certain rights in this invention.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a divisional of application Ser. No. 918,284 filed on Oct. 10,1986, now U.S. Pat. No. 4,776,999.

BACKGROUND OF INVENTION

1. Field of Invention

This invention relates to polymer films. More particularly, thisinvention relates to ultrathin polyether sulfone polymer films and theirproduction.

2. Prior Art

The preparation of various polymer films on a liquid support, such aswater, is known. For example, U.S. Pat. No. 2,760,233 discloses aprocess for preparing curved sheets of synthetic resins by floatingmonomeric or prepolymeric synthetic resins on a liquid surface. Amongthe numerous resins listed as being suitable in the patent's process arepolysulfones. However, no film thicknesses are disclosed.

In U.S. Pat. No. 4,008,203 and British Pat. No. 1,016,245, polyethersulfone films are disclosed. However, no film thicknesses are described.

In U.S. Pat. No. 4,108,837, polyether sulfone films having thicknessesof 1.0 mil or more are disclosed. However, the films are not ultrathin.

In U.S. Pat. No. 3,933,561 a process for manufacturing polymer films ona liquid surface is disclosed. Films with a thickness of less than onemicron, for example, 0.1 microns (i.e. 1,000 angstroms), are reportedlyprepared by the patent's process. The preferred class of polymers ispolysulfones. However, suitable chlorinated solvents for preparingultrathin, pinhole-free, polyether sulfone films having thicknesses of400 angstroms or less are not specifically disclosed.

U.S. Pat. No. 3,551,244 discloses a process for preparing on a watersurface films having a thickness between 0.05 and 5.0 microns (i.e.between 500 and 50,000 angstroms). The patent lists certain halogenatedsolvents, such as chloroform, as suitable solvents and states thatpolysulfone films can be made. However, suitable chlorinated solventsfor casting pinhole-free, polyether sulfone films are not specificallydisclosed.

U.S. Pat. No. 3,767,737 discloses a method for producing on a supportliquid nonporous polymer membranes having thicknesses between 0.005 and0.05 mils. Any polymer or copolymer capable of being cast as a film fromsolvents is allegedly suitable for use in the patent's process. However,polyether sulfone is not listed as one of the typically suitablepolymers and appropriate chlorinated solvents for preparingpinhole-free, polyether sulfone films are not specifically disclosed.

U.S. Pat. Nos. 4,155,793, 4,279,855 and 4,374,891 disclose processes forpreparing substantially void-free, ultrathin, permeable polymericmembranes of 500 angstroms or less. Natural or synthetic polymers areallegedly suitable for use in the patent's process. However, only filmsprepared from organopolysiloxane polycarbonate interpolymers mixed withpolyphenylene oxide are disclosed in the examples. Polyether sulfonefilms are not disclosed.

Other patents, such as U.S. Pat. Nos. 2,631,334, 2,689,187, and4,393,113 also disclose ultrathin polymeric films. However, no polyethersulfone films are disclosed.

In the prior art, the preparation of ultrathin, pinhole-free, polyethersulfone, free-standing films with thicknesses of less than 400 angstromsgenerally has not been disclosed. Usually, prior art polymer films withthicknesses of less than 400 angstroms that are cast contain holes orother macroscopic defects.

Accordingly, it is an object of this invention to prepare ultrathin,polyether sulfone polymer films.

It is a further object of this invention to prepare pinhole-free,polyether sulfone polymeric films.

It is also an object of this invention to prepare free-standing,polyether sulfone films having thicknesses of 400 angstroms or less.

These and other objects are obtained by the products and process of thepresent invention.

SUMMARY OF INVENTION

The instant invention provides a process for preparing macroscopicallypinhole-free, ultrathin, polyether sulfone films having thicknesses ofabout 400 angstroms or less. The films are prepared by dissolving apolyether sulfone polymer in 1,2,3-trichloropropane or1,1,2,2-tetrachloroethane to form a polymeric solution, casting thesolution on water to form a free-standing film, and removing the filmfrom the water. The ultrathin films of the present invention can be usedin separatory applications and as drug release membranes to facilitatethe controlled release of drugs.

DETAILED DESCRIPTION OF INVENTION

The preparation of polyether sulfones is well known. Generally,polyether sulfones are prepared by the ferric chloride-catalysedpolycondensation of monosulphonyl or disulphonyl chlorides. Suitablepolyether sulfones and their preparation are disclosed in "Synthesis ofPoly(arylene sulphones) by Polycondensation of Arylsulphonyl Chloridesunder Friedel-Crafts Conditions" by Cudby, et. al., Polymer 6, 589(1965), "Structures of the Poly(diphenylene ether sulphones) Obtained byPolysulphonylation," by Cudby et. al., Polymer 9, 265 (1965), BritishPat. No. 1,016,245 and U.S. Pat. Nos. 4,008,203, 4,105,636 and4,108,837, all of which are incorporated herein by reference.

The preferred polyether sulfones have the recurring units shown informulas I or II: ##STR1## wherein R is ##STR2## and the SO₂ group is inthe ortho, meta or para position on the ring, or ##STR3##

The polyether sulfone polymers that can be cast as macroscopicallypinhole-free, ultrathin films preferably have molecular weights in therange of about 5,000 to about 100,000 and most preferably about 20,000to about 50,000.

The most preferred polyether sulfone polymer has the repeating unitshown in formula II and can be prepared by nucleophilic substitution ofan aromatically bound halogen by phenoxy ions according to the followingreaction: ##STR4##

The most preferred polymer may also be prepared by a nucleophilicsubstitution in the following copper-catalysed reaction: ##STR5##

The most preferred polymer is commercially available from ImperialChemical Industry under the tradename "Victrex PES 5200G".

In order to prepare a polymer casting solution, the polyether sulfonepolymer is dissolved in a chlorinated solvent, preferably1,2,3-trichloropropane or 1,1,2,2-tetrachloroethane. The castingsolution is prepared by stirring the polymer and solvent at atemperature of less than 100° C. (eg. 60° C.) for several hours (eg.three to seven hours). The casting solution should contain about two toabout twelve percent by weight of the polymer based upon the totalweight of the casting solution, preferably about four to about eightpercent, and most preferably about six to about seven percent.Generally, the greater the amount of polymer in the casting solution,the thicker the films that are prepared. Conversely, the lower theamount of polymer, the thinner the films will be. However, if the amountof polymer is too low, such as one percent by weight or less, the filmwill be too fragile to lift from the casting surface. It is preferred toemploy the polymer solution immediately after preparation because, uponstanding for a few days, the solution tends to gel.

Cast polyether sulfone films can also be prepared from a mixed polymercasting solution containing a polyether sulfone polymer and a minoramount of other polymers which are compatible in film form with thepolyether sulfone and which can be dissolved in the casting solution.When other polymers are added to the casting solution, the amount ofpolyether sulfone employed should be 80 percent or more by weight basedupon the total weight of polymers dissolved in the polymeric castingsolution.

Before the polymeric solution is cast into films, it is preferred tofilter the solution using microfilters and/or membranes. Filtration ofthe polymer solution before casting substantially reduces imperfections,such as spots and pinholes, in the cast films. The solution can besuction filtered through glass microfiber filters and then passedthrough one or more membranes having pores with diameters of about oneto three microns. These filters are available from the MilliporeCorporation. For example, it is preferred to pass the solution through aseries of Millipore membranes, the first membrane having a pore size of3.0 microns, the second of 1.0 micron and the last of 0.45 micron. Inorder to enable the solution to pass through the smaller pore sizemembranes, it is usually necessary to apply pressure. For example, aMillipore stainless 47 mm pressure holder operated at a pressure up to100 psi argon can be used. The amount of pressure applied will dependupon the viscosity of the solution and the pore size of the membrane.Enough pressure to force the solution through the membrane is needed.

After filtration, the solution is cast on water at or near roomtemperature. As used therein, the term "water" includes aqueoussolutions containing minor amounts (e.g., one percent or less by weightbased upon the total weight of the solution) or organic solvents (e.g.,lower weight alcohols) the presence of which does not adversely affectthe properties of the films cast on the solution. The addition of suchorganic solvents may facilitate the removal of the film from the water'ssurface. The water is contained in any suitable walled container. Forexample, an appropriate container is an aluminum container havingdimensions of 12"×12"×3". Preferably, the walls of the container aresloped outwardly at about a 20 degree incline to reduce reflectedsurface waves which can damage the film. Such waves are produced whenthe polymeric solution is placed on the water's surface or by aircurrents and external vibrations. Most preferably, the inside walls ofthe container are teflon coated so that films are less likely to stickto the sides of the container.

The polymeric solution is cast by depositing a drop of the polymersolution upon the water's surface. The solution usually spreads over thesurface of the water in three seconds or less. The solution is allowedto stand until a sufficient amount of the solvent has evaporated to forma free-standing film. As used herein, the term "free-standing film"refers to a film that has a physically stable shape and is dimensionallystable on its casting surface and can be removed from the castingsurface without having to be supported over the entire surface area ofthe film. The time of evaporation generally is between 20 and 30 secondsand rarely more than about 60 seconds.

After the solvent has evaporated, the film is lifted from the liquidsurface using any suitable means, such as a 2"×3", thin, aluminum platehaving a 30 millimeter inner diameter hole in it and a handle on one endof the plate. When the aluminum plate touches the surface of the film,the film adheres to the aluminum plate and may readily be removed fromthe surface of the water.

The films of the instant invention are generally round, ultrathin,pinhole-free and uniform and have a thickness of about 400 angstroms orless, usually about 100 to about 180 angstroms. As used herein, the term"ultrathin film" refers to a film having a thickness of 400 angstroms orless, and the term "pinhole-free film" refers to a film having nomacroscopic holes.

The films of this invention can be used as gas separation membranes andin end uses where a controlled release of drugs is needed.

The invention is illustrated by the following examples in which arepercentages are by weight unless otherwise specified.

EXAMPLE 1

A polymer solution containing seven percent by weight polyether sulfonedissolved in 1,2,3-trichloropropane was prepared by magneticallystirring the solvent and the polymer for five hours at 60° C. Thepolyether sulfone polymer was Victrex PES 5200G which is available fromImperial Chemical Industries. Victrex PES 5200G is a high viscosity,amorphous, polyether sulfone having a Tg of 230° C., a flexural modulusof 3.7×10⁵ psi at 20° C., a tensile strength of 1.22×10⁴ psi at 20° C.and a repeating unit of: ##STR6##

After the polymer was dissolved in the solvent, the polymer solution wasfiltered through three microfiltration membranes obtained from theMillipore Corporation. A Millipore stainless 47 millimeter pressureholder operated at a pressure sufficient to force the solution throughthe membranes was employed. The first and second membranes had poresizes of 3.0 microns and 1.0 micron, respectively, and were Fluoroporepolytetrafluoroethylene membranes. The third membrane was a Duraporepolyvinylidene fluoride membrane and had a 0.45 micron pore size. Afterfiltration, a drop of the polymer solution was deposited on watercontained in a square aluminum container measuring 12"×12"×3" and havingteflon coated walls which were sloped away from the center at a 20degree incline. The drop spread rapidly over the surface of the water toform a film having a diameter of about five inches. After 30 seconds,the film was lifted from the surface of the water using a 2"×3" aluminumplate having a 30 mm diameter hole in the middle and a handle attachedat one end. The film was colorless and had a thickness of about 180angstroms. There were no macroscopic pinholes in the film.

EXAMPLE 2

Example 1 was repeated except that the polymer casting solutioncontained about five percent by weight polyether sulfone. The film had athickness of about 130 angstroms and contained no pinholes observable tothe eye.

EXAMPLE 3

Example 1 was repeated except that the polymer was dissolved in1,1,2,2-tetrachloroethane. The film contained no macroscopic pinholes.

As can be seen, both 1,2,3-trichloropropane and1,1,2,2-tetrachloroethane are suitable solvents from which ultrathin,pinhole-free polyether sulfone films may be prepared.

What is claimed is:
 1. A polyether sulfone free-standing polymer filmcontaining no macroscopic holes and having a thickness of about 180angstroms or less and randomly oriented molecules.
 2. The film of claim1 wherein the film is prepared from polyether sulfone polymers havingrepeating units of the formula: ##STR7## where R is ##STR8##
 3. The filmof claim 1 wherein the film is prepared from a polyether sulfone polymerhaving the general formula: ##STR9##
 4. The film of claims 1, 2 or 3wherein the polyether sulfone polymer has a molecular weight, M_(w), inthe range of about 5,000 to about 100,000.
 5. The film of claim 4wherein the molecular weight, M_(w), is in the range of about 20,000 toabout 50,000.
 6. The film of claims 1, 2 or 3 wherein the film has athickness in the range of about 100 to about 180 angstroms.
 7. The filmof claims 1, 2 or 3 wherein the film is prepared from a casting solutioncontaining a polyether sulfone polymer at about the 80 to about the 100percent by weight level and another polymer or polymers at about the 20to about the 0 percent by weight level based upon the total weight ofthe polymers dissolved in the casting solution.
 8. A polyether sulfonefilm containing no macroscopic holes and having a thickness of about 400angstroms or less and randomly oriented molecules prepared according tothe process comprising:(a) preparing a casting solution containing apolyether sulfone polymer at about the two to about the twelve percentby weight level, based on the total weight of the casting solution, in1,2,3-trichloropropane or 1,1,2,2-tetrachloroethane; (b) depositing thecasting solution on water to form a free-standing film; and (c) removingthe film from the water.
 9. A polyether sulfone film containing nomacroscopic holes and having a thickness of about 400 angstroms or lessand randomly oriented molecules prepared according to the processcomprising:(a) preparing a casting solution containing a polyethersulfone polymer having the repeating formula: ##STR10## present at aboutthe two or about the twelve percent by weight level, based upon thetotal weight of the casting solution, in 1,2,3-trichloropropane or1,1,2,2-tetrachloroethane; (b) depositing the casting solution on waterto form a free-standing film; and (c) removing the film from the water.10. The film of claim 8 wherein the polyether sulfone polymer has therepeating formula: ##STR11## wherein R is ##STR12##
 11. The film ofclaim 8 wherein the polymer has the repeating formula: ##STR13##
 12. Thefilm of claim 8, 9, 10 or 11 wherein the polyether sulfone polymer ispresent at about the four to about the eight percent by weight level inthe casting solution based upon the total weight of the castingsolution.
 13. The film of claim 12 wherein the polymer is present atabout the six to about the seven percent by weight level.
 14. The filmof claim 8 or 9 wherein the polyether sulfone polymer has a molecularweight, M_(w), in the range of about 5,000 to about 100,000.
 15. Thefilm of claim 14 wherein the molecular weight, M_(w), is in the range ofabout 20,000 to 50,000.
 16. The film of claim 8 or 9 wherein the castingsolution contains a polyether sulfone polymer at about the 80 to aboutthe 100 percent by weight level and another polymer or polymers at aboutthe 20 to about the 0 percent by weight level based upon the totalweight of the polymers dissolved in the casting solution.
 17. The filmof claim 8 or 9 wherein the film has a thickness in the range of about100 to about 180 angstroms.